This invention relates to a sensor device for determining the concentration of a substrate in a sample medium, comprising a membrane-covered amperometric enzyme electrode and a reference electrode, both of which are in contact with a flow measuring cell.
In many applications of medicine or biotechnology the accurate determination of a particular substrate concentration in a sample medium, e.g., the concentration of glucose in a perfusion solution, is of prime importance. For such purposes electrochemical sensors are used frequently, whose basic measuring principle will be explained below, using the determination of .beta.-D-glucose as an example.
According to common practice amperometric methods using a constant polarisation voltage Up are particularly well suited for substrate determination. In such methods .beta.-D-glucose is converted within a membrane containing the enzyme glucose oxidase (GOD) in immobilized form, according to the reaction equation given below; ##STR1##
The hydrogen peroxide formed during this process may be detected via anodic oxidation at a platinum electrode: EQU H.sub.2 O.sub.2 .fwdarw.O.sub.2 +2H.sup.+ +2e.sup.- EQU Up=600 . . . 700 mV
The intensity of the sensor current depends on the concentration of glucose in the sample solution.
Amperometric determination of glucose with the use of enzyme sensors is practicable only if both certain chemical and geometrical requirements are fulfilled. For example, the surface of the anode must be large enough to ensure sufficient sensitivity of the sensor. A large surface also is required for the reference electrode, by means of which the electric circuit is closed. A large, non-polarizable reference electrode will permit unimpeded collection of the sensor current.